1. Field of Invention
This invention is related to image processing systems. In particular, this invention is directed toward a system and method that determines a spectral curve from a color sensor.
2. Description of Related Art
A typical spectrophotometer measures the reflectance of an illuminated object of interest over a plurality of light wavelengths. Typical prior spectrophotometers in this context used 16 or higher channels measuring from approximately 400 nm to 700 nm, to cover the visible color spectrum or wavelength range. A typical spectrophotometer gives color information in terms of measured reflectances or transmittances of light, at the different wavelengths of light, from the test surface. This measures more closely estimates what the human eye would see as a combined image of a broad white light spectra image reflectance. This spectrophotometer desirably provides distinct electric signals corresponding to the different levels of reflected light received from the respective different illumination wavelength ranges or channels.
However, it is not efficient to install a reference spectrophotometer in every image processing system. As an alternative, a multiple light emitting diode (LED) reflectance spectrophotometer, such as that described in copending U.S. Ser. No. 09/535,007, incorporated herein by reference in its entirety, illuminates a target with a narrow band or monochromatic light. For a low cost implementation of the color sensor based on a multiple illuminant device as the illumination source, generally, for example, 10, 12 or 16 LEDs are selected. Each LED is selected to have a narrow band response curve in the spectral space. Therefore, for example, ten LEDs would correspond to ten measurements in the reflectance curve, when the measurements are obtained by measuring each LED independently. The LEDs, or other multiple illuminant based color sensor equivalent, e.g., lasers, are switched on one at a time as, for example, the measured media is passed through a transport of a printer. The reflected light is then detected by a photodetector and the corresponding voltage integrated. The integrated voltage is a function of the surface reflectance of the sample. These voltages are then normalized with, for example, a white tile. The normalized voltages are then converted to reflectance values to generate a fully populated reflectance spectra which conforms to standards set by, for example, the industry recognized reference spectrophotometer produced by Gretag Industries, hereinafter xe2x80x9cthe Gretag devicexe2x80x9d or comparable devices produced by the X-Rite corporation.
One way to achieve a full spectrum of measurements is to increase the number of illumination devices on the color sensor device. However, the addition of each additional illumination device increases the cost of the sensor and, since the measurements are taken serially, reduces the speed at which test measurements can be taken.
Therefore, the system and method of this invention use an algorithmic approach to convert a number of multiple illuminant sensor voltages to reflectance values independently of the response curve of the illuminant devices. For an alternative approach to determining a spectral curve see copending U.S. application No. 09/621,860, filed herewith, and incorporated herein by reference in its entirety.
In particular, a limited number of voltage measurements are received from, for example, a LED color sensor at the direction of a color sensor controller. The received sensor voltages are normalized in accordance with a calibration look-up table. The calibration look-up table contains the white tile measurements which are a standard practice and are well known in the color measurement industry. Next, the normalized sensor voltages are converted to reflectance values based on a correction look-up table obtained at mean LED wavelengths. Then, a reconstruction look-up table is used to convert the reflectance values at a mean wavelength to reflectance values with a predetermined wavelength separation to produce a full spectra reflectance curve output.
This invention provides a system and method that determine a full spectral reflectance curve.
This invention separately provides a system and method that determine a spectral curve using measurements from a switched multiple illuminant color sensor.
This invention separately provides a system and method that determine a spectral curve using measurements from a switched multiple LED color sensor.
This invention additionally provides a system and method in which reconstruction techniques are used to build transformation look-up tables.
These and other features and advantages of this invention are described in or are apparent from the following detailed description of the preferred embodiments.